The invention relates generally to plasma sources. In particular, the invention relates to liners used in inductively or microwave powered plasma sources.
Plasma processing is widely used in many applications in the field of fabrication of semiconductor integrated circuit. In the most common and long standing class of application, one or more wafers are inserted in a plasma reactor, and a processing gas is injected into the reactor and is excited into a plasma by coupling electrical energy into the plasma reactor. In etching applications, the processing gas typically includes a halogen-based gas, and typically RF power is applied to the pedestal electrode supporting the wafer to excite the gas into a plasma. In applications involving chemical vapor deposition, the processing gas includes chemical precursors of the material to be deposited, and typically RF power is applied to a showerhead electrode in opposition to the wafer being coated. The RF plasma activates the chemical reaction converting the precursor gas into the material coated on the wafer. In sputtering applications, typically negative DC power is applied to a target electrode comprising the material to be sputter deposited on the wafer. A working gas, such as argon, is excited into a plasma, and the positive argon ions are attracted to the negatively biased target to sputter the target material, which is then deposited on the wafer. Recently, there has been much interest in high-density plasma (HDP) reactors additionally including an RF inductive coil positioned adjacent to the plasma reactor to couple RF energy into a plasma source region.
Aside from these standard applications, plasmas have also been used for auxiliary purposes in semiconductor processing chambers. Plasmas are used to dry clean surfaces of the chamber without the need to open the chamber for operator access or even to vent the chamber to atmospheric pressure. Plasmas have also been used to clean or precondition wafer surfaces before the more standard types of processing, whether by plasma or by thermal activation.
Most typically, the plasma is generated in the processing chamber containing the substrate being processed. However, for some processes, the gas is excited into a plasma in a remote location and then transported in its excited state to the processing chamber. One such configuration is illustrated schematically in FIG. 1. A processing reactor 10 includes a pedestal 12 for supporting a wafer 14 to be processed. A vacuum pumping system 16 connected to the reactor 10 maintains the reactor at the relatively lower pressures associated with semiconductor processing, particularly plasma processing. These pressures are typically in the range of about 1 milliTorr to a few hundred Torr. The details of the reactor 10 are not illustrated, and the reactor may be configured for etching, CVD, sputtering, or possibly other processes. Particularly for CVD, the deposition may be performed by a thermal process while auxiliary functions may be performed by a plasma process.
A remote plasma source (RPS) 20 is connected to the reactor 10 but is distinctly separate from it. The RPS 20 receives gas from a gas source 22, excites it into a plasma, and delivers the plasma-excited gas to the reactor 10. A plasma usually contains some combination of ions and radicals of the excited gas. For example, a hydrogen plasma created from H2 gas may contain positively charged H+ ions and neutral H* radicals. Usually, the path between the remote plasma source 20 and the reactor 10 is long enough that the ions recombine before reaching the reactor and a mostly neutral stream of radicals is delivered to either process the wafer 14 or to clean the wafer 14 or the reactor chamber 10. However, there are applications, such as metal etching, in which a remote plasma source excites the processing gas into a plasma for etching or other direct processing of the wafer without the use of a plasma source within the reactor chamber.
Remote plasma sources usually rely on a large amount of microwave or RF energy applied to a dielectric tube carrying the gas. This configuration is referred to as an applicator. The large amounts of applied power, its application in sequences lasting on the order of minutes or less, and the corrosive nature of even argon plasmas have imposed severe design constraints on the applicator.
A similar set of problems has arisen in a structurally similar application of abatement plasma chambers. Semiconductor processing often involves noxious processing gases or noxious gaseous byproducts. In the past, the standard procedure has relied upon tall smokestacks to vent the gaseous exhaust from semiconductor processing reactors to a height sufficient that the noxious exhaust presents little risk of harm at ground level. Many people will no longer accept such a solution, and such exhausts are regulated by state and federal regulations. Chlorofluorocarbons (CFCs) have been shown to deplete the ozone layer on a world-wide scale. An international treaty has attempted to reduce if not virtually eliminate the emission of CFCs into the atmosphere. Furthermore, environmental and local political groups have shown an increasing intolerance for any emission of noxious material into the atmosphere regardless of the level of risk associated with such emission.
For these reasons, clean semiconductor processing systems are greatly desired. The use of CFC precursors has in large part been eliminated. Nonetheless, the complex plasma chemistry may result in a large number of materials in the exhaust, and the variety and uncertainty have made it difficult to assure that there is no noxious material in the reactor exhaust. Therefore, one favored approach scrubs the exhaust to somehow remove or deactivate those chemicals considered to be dangerous. One generic approach is to plasma treat the exhaust from the chamber to assure that the contents of the exhaust are in a benign form. For example, they have been thoroughly oxidized. With few exceptions, oxides of almost all materials used in semiconductor processing are not considered to be particularly dangerous.
Such a plasma abatement system is schematically illustrated in FIG. 2. It includes a dielectric plasma tube 30 positioned between the processing chamber 10 and the final stage of the vacuum pumping system 16. Additional pumping elements may be positioned upstream of the plasma tube 30, but the pressure within the tube 30 must be low enough to allow a plasma to be excited from the exhaust gas. An RF coil 32 is wrapped around the plasma tube and is powered from an RF source 34 to couple sufficient electrical energy into the plasma tube 30 to excite the gas within it into a plasma. A oxygen source 36 is positioned upstream of the plasma tube 32 to inject oxygen into the exhaust stream so that the plasma within the plasma tube contains not only the exhaust but also sufficient oxygen to oxidize substantially all of the oxidizable components of the exhaust whatever their source and composition. The figures fail to illustrate the valves and mass flow controllers associated with the gas sources 22, 36, 42.
To provide some specificity to the example, the reactor is assumed to be a capacitively coupled oxide etch reactor in which the pedestal electrode 12 is selectively powered from an RF power source 40 to excite a perfluorocarbon etching gas supplied from a source 42 into an etching plasma. Examples of perfluorocarbons are CF4, C2F6, C3F8, C4F8, and C4F6. Typically, an argon diluent gas is also supplied, but argon is a chemically inactive gas considered to be harmless in the small amounts associated with plasma processing and will not be further discussed. Although the perfluorocarbon is excited into a plasma to chemically react with the oxide being etched and to a lesser extent with the chamber parts and other parts of the integrated circuit structure, a substantial fraction of unreacted perfluorocarbon gas is exhausted from the etch chamber. Perfluorocarbons are not greatly toxic or poisonous, but they are considered to be harmful and corrosive. The purpose of the abatement plasma chamber is to oxidize PFCs into CO2 and COF2, both of which are gases considered to be relatively benign and not needing further cleaning or controlled disposal.
In the past, plasma tubes, as well as applicators, have typically been cylindrically shaped and composed of alumina ceramic. The ceramic tube must provide two functions. It operates as a vacuum wall for maintaining the tube interior at sufficiently low pressure for supporting a plasma. Its vacuum sealing capability is generally sufficient for a commercially sized vacuum pump to be able to maintain a vacuum down to at least 1 Torr. The ceramic tube must also act as dielectric window for passing microwave power (in the case of some applicators) or RF inductive power from an externally placed inductive coil into the tube interior. The electrical resistance of the tube wall must be high enough at the electromagnetic frequencies being used that the wall passes pass substantially all the radiation and absorbs very little power, certainly less than 10% of the incident power. These ceramic tubes typically have had diameters of about 2 inches (5 cm) and wall thicknesses of between {fraction (1/16)}xe2x80x3 to xe2x85x9xe2x80x3 (1.5 to 3 mm). A sufficient plasma density is achieved when the abatement plasma chamber is supplied with greater than 1 kW of RF power, typically in the frequency range of 2 to 13.56 MHz.
Such a design, however, has presented some difficulties. The nature of a high-power RF inductive coil is that a substantial DC voltage develops across it from one end to the other because of the resistive loss of the coil material. It is not uncommon for 1000 V of DC voltage to develop on the coil adjacent to the plasma tube. Such voltages will cause ions in the plasma to be accelerated to the inside of the plasma tube. The ion energy is dissipated as heat in the tube, greatly heating it, and some localized sputtering of the tube interior is inevitable at such high energies. Furthermore, ceramics tend to be poor thermal conductors and to be relatively fragile. The sudden application of RF power to the plasma tube creates thermal shock in the tube. The shock is intensified because the heating is localized to the area of the RF coil, and the heat only slowly diffuses to unheated areas. It is believed that these mechanisms are the source of cracking observed in the portions of the alumina tube directly underlying the coil wraps.
It is of course desirable to reduce if not eliminate the problems associated with high levels of RF power coupled through ceramic tubes, particularly for abatement plasma chambers.
Another concern with abatement plasma chambers is that the exhaust gas may contain a large number of chemical species. It is greatly desired that the oxidation of these materials be as complete and universal as possible, regardless of the pollutant composition.
The invention may be summarized as an inductively coupled plasma tube, particularly one used in a abatement plasma system, in which a porous ceramic insert is included on the inside of the ceramic tube forming both a dielectric window and a vacuum wall.
In another aspect of the invention useful in plasma abatement systems, a catalyst such as platinum or nickel is embedded in the porous ceramic.